Method and apparatus for determing the thickness of a chromium depleted zone of a surface region of a steel member

ABSTRACT

The invention provides a method and apparatus for monitoring subsurface chromium depletion from a steel member, such as a pyrolysis pipe. In the harsh conditions of a pyrolysis furnace, chromium within the pipe  16  migrates towards the pipe surface which results in the formation of a chromium depleted layer  14.  This layer can provide useful data about the condition and operation of the furnace. The degree of chromium depletion is measured by using a magnetic source of known strength to create a magnetic field in the surface region of the pipe  16.  An estimate of the thickness of the chromium depleted layer  14  is determined from the resultant magnetic flux, which can be measured by a hall element arranged at substantially 45° to the longitudinal axis of the magnet.

The present invention relates to a method and apparatus for makingmetallurgical tests. Although the invention is generally applicable, itis of particular application in the context of austenitic steels used inharsh environments, for example in the long pipes found in the pyrolysissection of petroleum cracking plants.

In a petroleum cracker, hydrocarbon molecules such as ethane and propanefrom natural gas, or heavier liquids such as naphtha and gas oil frompetroleum are split into smaller molecules. This is often done toprovide olefins such as ethylene that are useful in themselves, or maybe used in polymerisation processes.

In the case of ethane and propane, the gas is heated to above about 800°C. at which point bonds within the molecule break, producing a range ofsmaller molecules. The desired products are then separated out. The sameprinciple applies when cracking heavier substances, but since themolecules are much larger, a far greater range of smaller molecules isprovided. Although such processes provide a smaller yield of olefins,many other useful by-products are produced.

In a typical ethane cracker plant, the cracking takes place in apyrolysis section. Here, ethane is pumped through a maze of 4-6 inchdiameter tubes located within a furnace. This is essentially a largefirebox containing a large number of gas burners that are carefullyarranged and directed to provide even heating of the tubes. As theethane flows through the tubes it is heated up to about 800° C. andcracks. The ethane never comes into direct contact with the burnerflames; if it were to do so it would ignite disastrously.

Ethane is pumped through the pyrolysis section at a very high rate. Theresidence time of any individual molecule is a few seconds or less inolder plants and less than a one tenth of a second in more modernplants. It is important that the flow rate is kept this high in order toprevent the cracking process from running away. If it were to do so, theethane would crack not into the desired products, but into methane oreven carbon (coke) and hydrogen. A further measure that is taken tocontrol the possibility of runaway is the mixing of steam with theethane before it is fed to the furnaces. This has two beneficialeffects. The first is to lower the temperature necessary for thecracking to take place and the second is to reduce the amount of cokeformed and deposited on the inside of furnace tubes.

It will be appreciated that the combination of steam, hydrocarbons andvery high temperatures poses a significant safety problem. As a result,great efforts are made to design plants that are safe, and once thesehave been constructed they must be maintained in a safe and usefulcondition. In particular, it is of the greatest importance that regularchecks be made of critical components in the system. Having said that,modern petrochemical cracking plants are designed to produce vastquantities of product at a very high rate. The effect on theprofitability of a plant of the necessary downtime when carrying outthese vital safety checks is significant. There is therefore a greatincentive for these checks to be carried out expeditiously.

As noted above, the pipes in the pyrolysis section carry a largequantity of highly reactive chemicals that must be kept at hightemperature whilst being safely isolated from the heat source of thefurnace. Although they are made of specialist steel, as noted above,they are operating in a very harsh environment involving high mechanicalstress levels,.hash environment gases and gas composition variationswhich may, over time, lead to metallurgical problems. It is thereforeimportant to check the integrity of the pipes, e.g. to spot excessivecorrosion damage. In addition, it is,important to ensure that theburners are correctly aligned in order to ensure efficient operation ofthe furnace. Furthermore, incorrectly aligned burners may cause “hotspots” on the tubes that may result in local carburization and creepdamage.

The steels used to form the pipes are usually austenitic. They rely onthe chromium inherent in them to react with oxygen to form a dense layerof chromium oxide and also spinell oxides. This serves to protect themetal from further oxidation, thereby preventing the steel fromcorroding and in turn damaging the integrity of the structure.

The steel typically used in the furnace of an ethylene cracker comprises25 per cent chromium, 35 per cent nickel, and 0.5 per cent carbon withthe balance being iron. In the body of the steel, Cr₂₃C₆ precipitatesout to form particles of high chromium content. The result of this isthat most of the volume of the steel has about 20 per cent chromiumcontent but with clusters of much higher chromium content locatedtherein. The Cr₂₃C₆ clusters tend to dissolve over time and the chromiumthen migrates to the surface where it oxidises to form the protectivesurface oxide layer mentioned above —Cr₃O₂.

One problem that arises is that the harsh conditions in a pyrolysisfurnace may cause a fast growing less protective oxide layer, oxidespallation and Cr evaporation. In some cases this type of corrosion willresult in thinning of the metal and thus a weakening of theconstruction.

However, in an ethylene cracker the problem of possible faulty burneroperation mentioned above is more significant. This is because faultyoperation can over time lead to carburization and/or local creep damageand thus,possible tube failure. It is therefore highly desirable to beable to determine whether such faulty operation is occurring.

Viewed from a first aspect the invention provides a method of monitoringthe condition and/or operation of a furnace comprising the step ofmeasuring sub-surface chromium-depletion from a steel member.

The invention is particularly applicable to steel members in a pyrolysisfurnace, but is useful in other harsh environments. In a pyrolysisfurnace of a petroleum cracker the invention is particularly useful aspart of the process of monitoring the state of the pipes through whichthe hydrocarbons flow, although it may be applied to other components.

The invention is based on the fact that the chromium flux towards thetube surface will, due to chromium evaporation, oxidation and oxidespallation, over time deplete an oxide sub-layer of chromium because thechromium diffusion in the alloy matrix is not very fast. The result ofthis is that, as well as a Cr₃O₂ layer being formed on the outside ofthe steel, a chromium-depleted layer is formed beneath the oxide layer.Whilst this depletion is not itself a particular problem, the inventorshave recognized that, because of its relationship to the production ofthe oxide layer, the chromium-depleted layer provides useful data aboutthe condition and operation of the furnace.

As excessive or rapid oxidation may result in pipe weakness, the abilityto monitor the oxide layer in this way is useful in itself. Thus, viewedfrom another aspect the invention provides a method of determining thestate of a surface oxide layer on a steel member by measuring the degreeof chromium depletion in a sub-surface layer of the member.

Moreover, the growth rate of the oxide layer gives a useful indicationof the condition of the pipe and the operating conditions of thereactor. A thick oxide layer, particularly one that is growing quicklyand is therefore not tightly bonded to the remainder of the pipe, is notonly an indication of pipe weakness but may also provide evidence offaulty burner operation leading to higher-than-desirable thermal load orgreater temperature variation imposed on the tube in the region of thethick oxide formation. Thus, the chromium-depleted zone providesinformation about the oxide layer which in turn gives useful informationabout the operating conditions of the furnace. Accordingly, a preferredform of the invention uses the information determined about chromiumdepletion to detect pipe weakness and/or determine whether burneroperation is satisfactory.

Although absolute measurements of chromium depletion are useful, inorder to detect hot-spots caused by faulty burner operation, it isuseful to compare chromium depletion levels from different areas of asteel member. Thus, preferably a number of measurements are taken atdifferent points on a tube, e.g. along its length, and these arecompared. An area where significantly higher than average depletionoccurs is likely to correspond to a hot-spot.

Furthermore, it is preferred that the measurement(s) of the chromiumdepletion be repeated at intervals of time in order to provideinformation about the variation of the degree of chromium depletion withtime.

The level of depletion and the depth of the chromium-depleted layer arerelated to the chromium loss rate from the surface and the growing rateof the oxide. This is so because the chromium flux towards the outersurface will generally decrease when the thickness of the oxide layerincreases and the chromium diffusion from the inner alloy matrix willincrease when the chromium level in the depleted zone decreases. Thus asteady state can become established where the chromium flux out of thedepleted zone equals the chromium flux into the depleted zone. In thiscase the depleted zone thickness expresses the chromium loss rate andnot the total amount of chromium lost in oxidation etc.

In some cases, for instance when there is an oxidation problem in thefurnace, the oxidation is continuously fast and the amount of chromiumlost to the surface is much higher than the (refill) chromium flux fromthe inner alloy matrix. In this case the level of chromium depletion isapproximately proportional to the total amount of chromium lost overtime (integrated rate). If all the chromium sticks to the surface as athick fast growing oxide (without spallation and chromium evaporation)one could in this specific case say that the level of chromium depletionis proportional to the oxide thickness. Thus, at least under theseconditions, the method of the invention preferably further comprises thestep of determining the thickness of oxide on the steel member from themeasured chromium depletion.

However the level of chromium depletion can also express somethingbetween the chromium loss rate and total chromium loss. Furthermore, thelevel of chromium depletion is strongly dependent of material quality.

Clearly, it is highly undesirable to have to make destructive tests suchas cutting through the steel in order to examine its structure.Therefore, the invention preferably comprises the use of anon-destructive method of testing to measure chromium depletion.

Although other techniques could be used, it is preferred that the degreeof chromium depletion is determined by taking advantage of the variationof the magnetic properties of steel as its chromium content decreases.Thus, preferably, a magnetic source of known strength is used to createa magnetic field in the surface region and then an estimate of thethickness of the chromium-depleted zone is determined from the resultantmagnetic flux density at the surface of the member.

This measurement technique is believed to be independently inventive andso, viewed from another aspect, the invention provides a method ofdetermining the thickness of a chromium-depleted zone of a surfaceregion of a steel member comprising the steps of: using a magneticsource of known strength to create a magnetic field in the surfaceregion and then determining an estimate of the thickness of the chromiumdepleted zone from the resultant magnetic flux density at the surface ofthe member.

This technique takes advantage of the fact that Austenitic steel isgenerally paramagnetic at room temperature (i.e. it has a magneticpermeability of just above unity). However, if the chromium contentdrops below about 13-18 per cent, then the Curie point rises to ambienttemperature (in other words, the steel becomes ferromagnetic; it has ahigh magnetic permeability). The chromium-depleted layer has a chromiumcontent of below 13-18 percent and is therefore ferromagnetic.

Thus, the magnetic properties of steel containing significant amounts ofchromium are different from those containing a lesser amount. This hasthe result that, for a given magnetic field strength applied when themethod of this aspect of invention is carried out, the flux density insuch a region of the steel increases with increased chromium depletion.Therefore, by measuring the magnetic flux density, an indication of theamount of chromium depletion can be made and this in turn provides anestimate of the thickness of the chromium oxide layer.

By “measuring” it is not meant that a specific value of magnetic fluxdensity in any standard unit is necessarily provided, although thiscould be done if desired. Rather, an output measurement which varies ina predictable manner in relation to changes in flux density is required.As noted above, in many instances all that is needed is to be able tocompare measurements made at different places and/or times. However, ifdesired, the output may then be processed or suitably calibrated inorder to provide an indication of the chromium depletion

In order to calibrate the apparatus used, samples of steel with varyingdegrees of chromium depletion may be tested according to the method ofthis aspect of the invention. Subsequently, they may be cut up and thedegree of chromium depletion determined using a scanning electronmicroscope. Field strength can then be correlated to chromium depletionfor each type of steel tested. Since the magnetic properties of steelvary from one grade to another, it is desirable that for each grade thatis to be tested by the method of the invention, samples of known layerthicknesses are tested.

It should be noted that the relationship between measured field strengthand depletion-layer thickness is very non-linear. Indeed, for any testapparatus that is used, there will be a depletion-layer thickness thatresults in saturation.

For ease of measurement it is preferred that the magnetic source bearranged such that the flux density is measured at a position where themagnetic field lines are generally normal to the surface. It will beappreciated that as magnetic field lines are curved they cannot all benormal to the surface, but preferably the arrangement is such that thefield lines in question are as near to normal as practicable.

In most cases the geometry is such that this objective can generally beachieved by using a magnet having its axis aligned at between 30 and 60and more preferably between 40 and 50 degrees to the surface of thesteel member. Most preferably, the axis is at substantially 45 degreesto the surface of the steel member.

Although an electromagnet may be used to generate the magnetic field ofthe invention, it is most convenient to use a permanent magnet.Preferably a fairly strong but small bar magnet is used. Typicaldimensions will be between 0.5 and 20 mm, preferably around 10 mm inlength and 2 to 5 mm, preferably around 3 mm in width. A useful fieldstrength is between 300 and 400 milli-tesla, preferably around 350milli-tesla. However, it will be appreciated that stronger magnets areneeded where it is necessary to penetrate further into the steel.

Although any suitable means of measuring the magnetic flux may beapplied, such as a search coil, it is most convenient to use a Halleffect device such as a commercially available Hall element. Suchdevices are generally around 3 mm square and are available as integratedunits which are particularly convenient in the present application.Where the magnetic field lines are arranged to be generallyperpendicular to the surface of the steel it follows that the sensingdirection of the hall element should also be arranged to be normal tothe surface. Thus, in the standard integrated unit this means that thelarger base surface be applied or held proximate to the surface of thesteel member.

Preferably the Hall element and magnet are very close together, if nottouching. In the most preferred form of the invention the sensingdirection of the Hall element is arranged at 45 degrees to the axis ofthe magnet.

In order to secure the components together it is preferred that a smallhousing be provided such that the entire apparatus needed to perform asensing part of the invention forms a small and compact unit. Since inuse the unit will generally be moved across the surface of the steelmember it is preferred that a hard non-magnetic pad be provided underthe Hall element to prevent damage caused by friction. This shouldpreferably only be a millimetre or two in thickness to avoid reducingthe sensitivity of the device.

A Hall element or other sensor will provide an output voltage which maythen be converted into a suitable form to indicate the degree ofchromium depletion. In the simplest case, it may be fed directly to asuitably calibrated voltmeter to provide a direct reading. With a moresophisticated device, the output signal from the probe may be fed to acomputer, generally via an analogue-to-digital converter. A suitableprogram may be arranged to convert the input signal to a useful reading.This could perhaps be based upon first principles (i.e. a mathematicalmodel), but it will be more straightforward to interpolate from alook-up table based upon experimental tests. As noted above, there willbe a layer thickness where saturation occurs such that the outputvoltage from the Hall element reaches a maximum. Preferably means isprovided to indicate when this occurs. It is, of course desirable todesign the apparatus used such that saturation occurs at a depletionlayer thickness that is greater than that likely to be found.

It will be appreciated from the foregoing that the invention alsoextends to a dedicated apparatus for performing the invention andtherefore viewed from a further aspect the invention provides anapparatus for determining the thickness of a chromium depleted zone of asurface region of a steel member, the apparatus comprising a magneticfield source and a means for measuring the magnetic flux density,wherein the apparatus is arranged such that when it is placed proximateto a steel member the measuring means determines the magnetic fluxdensity in the surface region of the steel resulting from the magneticfield source.

Preferably, the invention further provides means for providing anindication of the chromium depletion based upon the flux density.

Furthermore, the apparatus is preferably arranged to operate inaccordance with one or more of the preferred forms of the method setforth above. In a particularly preferred form, the apparatus comprises ahall element having its axis arranged at substantially 45 degrees to thelongitudinal axis of a bar magnet. Preferably these components arecontained in a housing and furthermore it is particularly preferred thata protective pad be provided under the hall element. An output signalfrom the Hall element is preferably fed via an analogue-to-digitalconverter to a suitable computer as discussed above.

A preferred embodiment of the invention will now be described, withreference to the accompanying drawings, in which:

FIG. 1 is a partially sectional view of a probe according to theinvention;

FIG. 2 is a schematic block diagram illustrating the inter-connection ofthe probe and other components;

FIG. 3 is a generally schematic view illustrating the magnetic fluxlines in a non-depleted steel surface;

FIG. 4 is a view corresponding to FIG. 3 in which the chromium has beendepleted;

FIG. 5 is a graph illustrating the output voltage from three differentprobes according to the invention plotted against simulated depletionlayer thickness;

FIG. 6 is a graph illustrating the variation in output voltage from thesame sensors as in FIG. 5 as the distance between the probes and thedepletion layer is varied;

FIG. 7 is a graph illustrating the change in output voltage from asensor according to the invention caused by the presence of a simulatedlayer of carburized steel; and

FIG. 8 is a graph comparing the output voltages produced by a probeaccording to the invention when applied to two different grades of steelthat had been subjected to high temperatures.

With reference first to FIG. 1, probe 1 comprises a bar magnet of 10×3mm in size and having a strength of 350 milli-teslas. It is locatedagainst a Hall element 3 with its axis at 45 to the sensing axis of Hallelement. Hall element 3 is provided with a protective pad 4 at its basemade of non-magnetic metal.

These components are all located within a plastic housing 5 and areinterconnected by means of epoxy resin 6 which fills the void within thehousing. Conductive leads 7, 8 and 9 extend through the epoxy andthrough an opening in the upper portion of the probe. Lead 9 provides aDC input to drive the Hall element, lead 8 is common and lead 7 providesa signal output from the Hall element. The Hall element is a standardcommercially available integrated device. Leads 7, 8 and 9 willgenerally be provided within a flexible coiled cable such that the probecan be readily moved over a surface that is being tested.

As may be seen from FIG. 2, the probe 1 is connected via signal lead 7to an analogue to digital converter 10. Leads 8 and 9 are connected aspreviously mentioned. The digital output from converter 10 is then fedto a computer processor 11 in which software converts the signal into avalue corresponding to the thickness of the chromium depletion layer ofthe steel member being tested. This is done by means of the results ofempirical calibration tests whose results are stored in look-up tables.The processor may also be programmed to determine estimates of chromiumoxide layer thickness. This is then displayed on a suitable displaymeans 12.

As shown in the figure, components 10, 11 and 12 are convenientlyprovided together in a single portable integrated package 15 with theprobe 1 being separate and freely moveable in relation to it.Alternatively, the analogue to digital converter could be providedintegrally with, the probe and components 11 and 12 could be therespective parts of a standard personal computer.

FIG. 3 illustrates the probe 1 in position against a steel member 13which does not have a depleted chromium layer. As may be seen, where thechromium is not depleted the magnetic field lines are significantlyspaced apart as a result of the lower magnetic permeability of thesteel. In contrast to this, FIG. 4 illustrates a depleted layer 14 nearthe surface of steel member 16 which has a much higher magneticpermeability. This results in the magnetic field lines being closertogether in the same way as an iron core in an electromagnet.

It will be appreciated that by detecting the change in the magnetic fluxdensity measured by the Hall probe an indication of the level ofchromium depletion (which is proportional to depletion layer thickness)can be achieved. The voltage level on signal line 7 is predictablyrelated (non-linearly) to the thickness of the depletion layer.Consequently, once the signal has been converted to digital form it maybe processed in processor 11, for example by interpolating from asuitable experimentally-derived look-up table. The oxide layer thicknessis in turn related to the thickness of the depletion layer and may alsobe determined in a similar manner.

Finally, a suitable output, such as the estimated chromium depletionlayer thickness in microns and/or the raw output voltage from the probe,may be displayed on display unit 12.

In order to demonstrate the operation of the probes according to theinvention, a series of simulation-based experiments have been carriedout using ferromagnetic foils to simulate chromium-depleted layers.

Output voltage readings from three different probes (identified asprobes 1-3) are shown in FIG. 5 as a function of a ferromagnetic foilthickness. Here the probes were placed directly against foils varying inthickness. As may be seen, there is a significant and clearly detectablevariation in output voltage with thickness up to around 200 μm. Beyondthis the variation becomes significantly less and above around 500 μmsaturation occurs.

Since the ferromagnetic chromium-depleted layer is actually found belowthe surface of the steel, tests were also carried out to determine theeffect of moving the probe away from the ferromagnetic foils. Theinfluence from this so-called ‘lift off’ on the readings is shown inFIG. 6. Here the same three sensors were used to make measurements on athick ferromagnetic surface. The distance between the base of the probeand the ferromagnetic material was varied and the output voltagesrecorded. It may be observed that as the lift-off increases beyond about2000 μm, the output signal becomes significantly reduced. Nevertheless,it will be seen from FIGS. 5 and 6 that the probe is suitable forproviding useful data from typical chromium-depleted layers which arelocated just below the outer oxide layer.

In fact, at least in the context of a pipe in a pyrolysis furnace, it isundesirable for the probe to be affected by ferromagnetic layers deepbelow the surface because these may be caused by carburization of theinside surface of the pipe. A major reason for the actual geometricarrangement of the magnet and the Hall element shown in FIG. 1 is thedesire to minimize the interference from a magnetic carburized areadeeper into the material. FIG. 7 shows how the measured output voltagesfrom a probe placed on simulated 50 μm and 100 μm thick depleted zonesis influenced by a simulated carburized area. Ferromagnetic steelrepresenting carburized steel was located at varying distances beneathfoils representing a chromium-depleted layer. The indicated signalvoltage is the increase caused by the presence of ferromagnetic steel.One can see that in the case of the 100 μm depleted layer, when thecarburized area is located more than 2 mm underneath it the readingsfrom the probe (which was 200 mV without the “carburized” steel present)is increased only moderately by about 50 mV (to 250 mV). Therefore, forany useful pipe thickness it can be assumed that carburization of theinterior of the pipe should not cause significant inaccuracies in theoutput readings.

Finally, FIG. 8 illustrates the result of an experiment where a probeaccording to the invention was used to test a steel rod that had beensubjected to high temperatures.

In order to compare the output signals caused by different degrees ofoxidation on different grades of steel, the rod was welded together fromtwo halves made of different materials: 25/35 Cr/Ni and 35/45 Cr/Ni. Itwas exposed to cyclic oxidation for about 1000 hours in an oven having atemperature gradient of 600° C. to 1040° C.

The figure has one line to indicate the variation of temperature(degrees Celsius) with position in the oven. The other two linesindicate output voltages from a sensor placed at different positionsfirstly on a weld on the rod and secondly on the rod itself.

It may bee seen that the weld, which is symmetrically installed in thefurnace, gives a reading proportional to the temperature in both halvesof the furnace. The 25/35 Cr/Ni and 35/45 Cr/Ni materials also givereadings proportional to the local thermal load. However, the largedifference between 25/35 Cr/Ni and 35/45 Cr/Ni material is apparent.This indicates the difference in oxidation resistance between these twomaterials.

As discussed above, it would be necessary to calibrate the probe foreach material type before the readings from different alloys could becompared. However where the probe is used to identify irregularities inthe furnace operation (for example looking for “hot spots” caused bypoor burner set-up) the relative probe readings obtained from the samematerial type and age could be used. For example, a series ofmeasurements may be taken along the length of a particular pipe and/orat different places around its circumference.

If necessary, the movement of this probe over the surface of the steelmember may also be measured in a known manner and this, in combinationwith oxide layer thickness data may be used to provide a map on display12 showing the variation of the output voltage (or depletion layerthickness if calibrated) along the member.

1. A method of monitoring the condition and/or operation of a furnace comprising the step of measuring sub-surface chromium-depletion from a steel member.
 2. A method as claimed in claim 1, wherein the steel member is a pipe within a pyrolysis furnace through which hydrocarbons flow.
 3. A method as claimed in claim 1, further comprising the step of using the measurement of chromium depletion to estimate the state of a surface oxide layer.
 4. A method as claimed in claim 1, further comprising the step of using the measurement of chromium depletion to determining whether burners in the furnace are operating satisfactorily.
 5. A method as claimed in claim 1, wherein a magnetic source of known strength is used to create a magnetic field in the surface region of the steel member and an estimate of the thickness of the chromium-depleted zone is determined from the resultant magnetic flux density at the surface of the member.
 6. A method of determining the thickness of a chromium-depleted zone of a surface region of a steel member comprising the steps of using a magnetic source of known strength to create a magnetic field in the surface region and then determining an estimate of the thickness of the chromium depleted zone from the resultant magnetic flux density at the surface of the member.
 7. A method as claimed in claim 6, wherein the flux density is measured at a position where the magnetic field lines are generally normal to the surface.
 8. A method as claimed in claim 6, wherein the magnetic field is created in the surface region by a magnet having its axis at between 30 degrees and 60 degrees to the surface of the steel member.
 9. A method as claimed in claim 8, wherein the axis of the magnets is at substantially 45 degrees to the surface of the member.
 10. A method as claimed in claim 6, wherein the magnetic field is created by a permanent magnet.
 11. A method as claimed in claim 6, wherein the magnetic flux density is determined by a Hall-effect probe located proximate the surface of the steel member.
 12. A method as claimed in claim 11, wherein a hard non-magnetic pad is provided between the Hall-effect probe and the surface.
 13. A method as claimed in claim 11, wherein the field detection axis of the Hall-effect probe is aligned at substantially 45 degrees to the north-south axis of the source of the magnetic field.
 14. A method as claimed in claim 11, wherein an output signal from the Hall-effect probe is processed in order to provide a direct indication of the thickness of the chromium depleted zone and/or the thickness of an associated oxide layer.
 15. A method as claimed in claim 6, further comprising the step of determining an estimate of the surface oxide layer thickness.
 16. Apparatus for determining the thickness of a chromium depleted zone of a surface region of a steel member, the apparatus comprising a magnetic field source and a means for measuring magnetic flux density, wherein the apparatus is arranged such that when it is placed proximate to a steel member the measuring means, determines the magnetic flux density in the surface region of the steel resulting from the magnetic field source.
 17. Apparatus as claimed in claim 16, further comprising means to process the output from the measuring means and to display the thickness of the chromium depleted zone and/or the thickness of an associated oxide layer.
 18. Apparatus as claimed in claim 16 arranged to operate in accordance with the method of monitoring the condition and/or operation of a furnace comprising the step of measuring sub-surface chromium-depletion from a steel member. 